The invention relates generally to imaging sensors and more particularly to an imaging sensor utilizing CMOS active pixels.
Active Pixel Sensors (APSs) are utilized in various imaging devices, such as telescopes, digital cameras and video recorders. An APS captures an image of a scene of interest by converting incident light from the scene into electrical signals in an analog form. A typical active pixel sensor has an array of xe2x80x9cpixelsxe2x80x9d or discreet regions, each pixel containing a light-sensitive element. Each light-sensitive element in a pixel generates a separate electrical current, which is proportional to the intensity of the incident light on that element. The varying magnitude of this electrical current is used as a basis for conversion into a stream of digital image data by an analog-to-digital converter (ADC), which can be stored in memory. The digitized image data from all the pixels can then be displayed as a composite image on a monitor, printed onto a sheet of paper, or analyzed for information concerning the properties of objects in the scene.
The pixels that are utilized in conventional APSs can be classified into two types of pixels. The first type of pixel is commonly referred to as an xe2x80x9canalog pixel.xe2x80x9d An analog pixel includes a photo sensor, such as a photodiode or a phototransistor, and may include an amplifier. An associated ADC and memory are located external to the pixel. Therefore, any current generated by the photo sensor of an analog pixel is transmitted from the pixel to the external ADC as an analog signal.
The second type of pixel is known as a xe2x80x9cdigital pixel.xe2x80x9d A digital pixel includes not only a photo sensor and an amplifier, but also an ADC. In other words, the ADC is contained within the pixel, along with the photo sensor and the amplifier. Thus, the magnitude of current generated by the photo sensor is digitized within the pixel and can be transferred to off-pixel components as a digital signal. U.S. Pat. No. 5,461,425 to Fowler et al., entitled xe2x80x9cCMOS Imaging Sensor with Pixel Level A/D Conversion,xe2x80x9d describes an imaging sensor on a single semiconductor chip having pixels of the second type. The imaging sensor of Fowler et al. has an array of pixels, wherein each pixel includes a phototransistor and an ADC. The analog signal generated by the phototransistor is converted to a serial stream of digital data bits by the on-pixel ADC. The digital data is then filtered and stored in an external memory. The on-pixel ADC is described as having the advantage of minimizing parasitic effects and distortion caused by low signal-to-noise ratio.
The prior art active pixel sensors, regardless of the pixel type, operate to image a scene of interest by quantifying the degrees of radiance from various scene segments. For each scene segment, a particular pixel quantifies the degree of radiance from the scene segment by measuring a photo voltage driven by a photo-sensor-generated current. When a photo sensor is exposed to incident light from a segment of the scene for a fixed integration or exposure time period, the magnitude of a photo voltage will be dependent upon the intensity of radiance from the scene that is being imaged by the photo sensor.
FIG. 1 illustrates the technique utilized by the prior art imaging sensors to quantify the intensity of radiance from a scene segment. Referring to FIG. 1, three lines 10, 12 and 14 are plotted with respect to voltage over time. The lines 10, 12 and 14 represent photo voltages corresponding to three degrees of radiance from the scene segment that is sensed by a prior art imaging sensor. The time period from t=0 to t=T is the fixed exposure time period utilized by the imaging sensor. The line 10 represents the voltages over time when the degree of radiance from the scene segment is the maximum level detectable by the imaging sensor. The line 14, on the other hand, represents the voltages over time when the degree of radiance from the scene segment is at the minimum level detectable by the imaging sensor. Lastly, the line 12 represents the voltages over time when the degree of radiance from the scene segment is at the mean illumination level.
At the end of the fixed exposure period, i.e., t=T, the imaging sensor quantifies the magnitude of the photo voltage using an ADC. When the degree of radiance from the scene is at the detectable maximum level, the voltage equals VSAT, as indicated by the line 10 at t=T. At the mean illumination level, the voltage is VMEAN, as indicated by the line 12 at t=T. Lastly, at the detectable minimum level, the voltage is VRESET, as indicated by the line 14 at t=T. The imaging sensor configured to the limits defined by VSAT and VRESET will be able to differentiate discrete degrees of scene radiance that result in a photo voltage between VSAT and VRESET. However, the amount of differentiable degrees of scene radiance that can be detected by an imaging sensor is at least partially dependent on the resolution of the ADC. As another factor that affects image quality, the radiance sensitivity may be adjusted by shortening or extending the length of the fixed exposure period, but the adjustment is a tradeoff of increasing sensitivity of either high radiant scene segments or low radiant scene segments.
Although the prior art imaging sensors operate well for their intended purpose, what is needed is an imaging sensor having a superior imaging performance, as defined by its dynamic range, and a greater sensitivity to low radiant scene segments.
A system and a method for imaging a scene of interest determine a scene segment radiance based upon time periods (xe2x80x9cexposure periodsxe2x80x9d) required to achieve a fixed voltage drop. Thus, rather than the conventional technique of sampling the voltage level after each set exposure time (i.e., time-driven sampling), sampling data is based on the time required for a set voltage drop (i.e., voltage-driven sampling). The voltage-driven sampling occurs at each pixel in a pixel array that is used to provide image information in discrete scene segments. The rate of voltage drop corresponds to the intensity of scene segment radiance, such that high radiant scene segments yield more rapid voltage drops than lower radiant scene segments. The variable exposure periods are measured for each pixel in the pixel array to gather exposure periods from different segments of the scene being imaged. The measured exposure periods are then translated into grayscale information that can be used to generate a composite image having various levels of grayscale that are representative of the imaged scene.
The variable exposure period is measured within each pixel by comparing the voltage at a floating diffusion (FD) node to a reference voltage. The voltages at the FD node and the reference voltage are input to a comparator that outputs a signal when the voltage at the FD node is equal to or less than the reference voltage. The FD node is connected to a photo sensor, e.g., photodiode, to generate a photo current in response to incident light from an associated scene segment. The generated current causes dissipation of charge from an integration capacitor connected to the FD node, causing the voltage at the FD node to decrease. The rate of decrease in voltage at the FD node is accelerated if the magnitude of the photo current is increased. Therefore, the time period for the voltage at the FD node to drop from a reset voltage to the reference voltage is mathematically related to the radiance of the scene segment from which the pixel receives light energy. This time period defines the exposure period for the scene segment being imaged.
The duration of the exposure period is then digitized for signal processing. In the preferred embodiment, each pixel includes an analog-to-digital converter (ADC) to digitize the duration of the exposure period within the pixel. The ADC is designed to capture a digital count that is supplied by an off-pixel counter. The captured digital count represents the exposure period for the scene segment being imaged. The off-pixel counter may be configured to provide linearly progressing digital counts. However, the off-pixel counter may also be configured to provide non-linear digital counts. The non-linear digital counts may be utilized to change the relationship between the exposure period and the degree of radiance from a scene segment.
In this preferred embodiment, each pixel also includes memory to store the captured digital count. The in-pixel memory contains a number of memory cells that can store the captured digital count. Preferably, the number of memory cells is at least as great as the number of bits in the digital count. The memory cells are configured such that each memory cell is coupled to a bi-directional bit line. The bi-directional bit line functions as both a read bit line and a write bit line.
The memory cells are dual port memory cells having a three-transistor configuration. Each memory cell includes a write access transistor, a read access transistor, and a data-controlled transistor. The read access transistor and the data-controlled transistor are configured to provide a conductive path from an associated bi-directional bit line to ground. The write access transistor connects the bi-directional bit line to the gate of the data-controlled transistor. A storage node is located between the write access transistor and the gate of the data-controlled transistor. Thus, the data stored in the storage node can control the conductive state of the data-controlled transistor. During a read operation, the stored data in the storage node is indirectly read by the effect of the conductive state of the data-controlled transistor. Therefore, the stored data is not destroyed during the read operation. The non-destructive feature of the memory cell allows less frequent refresh cycles, or no refresh requirement for high speed read-out applications.
An advantage of the invention is that each pixel contains an electronic shutter in which all the pixels simultaneously sense, digitize, and store digital image data in response to a scene being imaged. Thus, the pixels output digital signals, rather than analog signals. In addition, the pixels can store the digital image data for an indefinite period, and thereby functions as an image frame buffer.
Another advantage of the invention is that a greater dynamic range is achieved, which equates to superior imaging performance. Still another advantage is that sensitivity for low radiant scene segments are increased with the use of variable exposure periods. Furthermore, the pixel design is compatible with scaled CMOS technologies with a low supply voltage.